The present invention relates generally to sputter deposition of thin films using rotating cylindrical magnetrons, and more particularly to mounting arrangements for such magnetrons.
DC reactive sputtering is the process most often used for large area commercial coating applications, such as the application of thermal control coatings to architectural and automobile glazings. In this process, the articles to be coated are passed through a series of in-line vacuum chambers isolated from one another by vacuum locks. Such a system may be referred to as a continuous in-line system or simply a glass coater.
Inside the chambers, a sputtering gas discharge is maintained at a partial vacuum at a pressure of about three millitorr. The sputtering gas comprises a mixture of an inert gas, such as argon, with a small proportion of a reactive gas, such as oxygen, for the formation of oxides.
Each chamber contains one or more cathodes held at a negative potential of about -200 to -1000 volts. The cathodes may be in the form of elongated rectangles, the length of which spans the width of the line of chambers. The cathodes are typically 0.10 to 0.30 meters wide and a meter or greater in length. A layer of material to be sputtered is applied to the cathode surface. This surface layer or material is known as the target or the target material. The reactive gas forms the appropriate compound with this material.
Ions from the sputtering gas discharge are accelerated into the target and dislodge, or sputter off, atoms of the target material. These atoms, in turn, are deposited on a substrate, such as a glass sheet, passing beneath the target. The atoms react on the substrate with the reactive gas in the sputtering gas discharge to form a thin film.
The architectural glass coating process was made commercially feasible by the development of the magnetically-enhanced planar magnetron. This magnetron has an array of magnets arranged in the form of a closed loop and mounted in a fixed position behind the target. A magnetic field in the form of a closed loop is thus formed in front of the target plate. The field causes electrons from the discharge to be trapped in the field and travel in a spiral pattern, which creates a more intense ionization and higher sputtering rates. Appropriate water cooling is provided to prevent overheating of the target. The planar magnetron is further described in U.S. Pat. No. 4,166,018.
A disadvantage of the planar magnetron is that the target material is only sputtered in the narrow zone defined by the magnetic field. This creates a "racetrack"-shaped sputtering zone on the target. Thus, a "racetrack"-shaped erosion zone is produced as sputtering occurs. This causes a number of problems. For example, (1) localized high temperature build-up eventually limits the power at which the cathodes can operate, and (2) only about 25 percent of the target material is actually used before the target must be replaced.
The rotary or rotating cylindrical magnetron was developed to overcome some of the problems inherent in the planar magnetron. The rotating magnetron uses a cylindrical cathode and target. The cathode and target are rotated continually over a magnetic array which defines the sputtering zone. As such, a new portion of the target is continually presented to the sputtering zone which eases the cooling problem, allowing higher operating powers. The rotation of the cathode also ensures that the erosion zone comprises the entire circumference of the cathode covered by the sputtering zone. This increases target utilization. The rotating magnetron is described further in U.S. Pat. Nos. 4,356,073 and 4,422,916, the entire disclosures of which are hereby incorporated by reference.
The rotating magnetrons, while solving some problems, presented others. Particularly troublesome has been the development of suitable apparatus for driving and supporting the magnetron in the coating chamber. Vacuum and rotary water seals have been used to seal around the drive shaft and cooling conduits which extend between the coating chamber and the ambient environment. However, such seals have a tendency to develop leaks under conditions of high temperature and high mechanical loading.
Various mounting, sealing, and driving arrangements for cylindrical magnetrons are described in U.S. Pat. Nos. 4,443,318; 4,445,997; and 4,466,877, the entire disclosures of which are also hereby incorporated by reference. These patents describe rotating magnetrons mounted horizontally in a coating chamber and supported at both ends. It is often preferable, however, to support the magnetron at only one end by a cantilever mount. However, the cantilever mounting arrangement produces the highest bearing loads. Several examples of cantilever mounted rotary magnetrons are given in Design Advances and Applications of the Rotatable Magnetron. Proceedings of the 2nd National Symposium of the American Vacuum Society, Vol. 4, No. 3, Part 1, pages 388-392 (1986), the entire text of which is hereby incorporated by reference.
Conventional bearings alone are not particularly suitable for mounting a rotary magnetron. Such bearings do not operate well in a vacuum environment, as their lubricants have too high a vapor pressure. Additionally, some sort of vacuum seal has to be provided.
Rotary vacuum seals have been used to seal around the drive shafts and the cooling conduits extending into the coating chamber. These seals must make contact with the drive shaft or conduits, and the aperture in the vacuum chamber through which the shaft and conduits pass. A rotary vacuum seal also may function as a bearing. High operating temperatures and bearing loads, however, can accelerate wear on the seals, causing vacuum leaks.
Rotary water seals have also been used on rotary magnetrons. However, they are very unreliable when operated in a vacuum. Specifically, these seals tend to break down after a relatively short period of operation.
Such factors have all contributed to the unreliability and leakage problems associated with the use of rotating magnetrons.
In view of the foregoing, an object of the present invention is to provide an improved mount for a rotating cylindrical magnetron.
A more specific object of the present invention is to provide a cantilever mount for a rotating cylindrical magnetron which will operate for extended periods without failure due to leaks in its vacuum seals and bearings.
Additional objects and advantages of the invention will be set forth in the description which follows or may be learned by practice of the invention. Other objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims.